Form blocks 02

IP
i From right, the
original Sonex
Fiberglass wingtip,
left and right halves
of flow-forming
blocks/dies, and
the tack-welded
wingtip of 5052
0.032-inch aluminum sheet.
A CRAFTSMAN HAS TIMES IN HIS
life when he j u s t has to
make something out of
sheet metal. It may happen after too much of the
domestic work of fabric
covering, or it might be
stimulated by the boredom of s a n d i n g primer,
catching up on paperwork,
c l e a n i n g the shop, or
painting the garden tools.
The itch is there. It must
be scratched.
Wingtips can be frustrating. This is a fact. They
Flow-Forming
Wingtips
Form Blocks, Part 2
KENT WHITE
do not have to be f a n c y
structural tips like those
on the Globe Swift, with
several stiffening beads on
both surfaces, stepped
edges, and interior ribs.
No, w i n g t i p s can be
fairly simple, like those on
a North American P-51
Mustang or even a Sonex,
and still be plenty interesting. First, they have to fit Pulling the concrete welding jig free makes me glad I
nice and tight against the used the wax and the optional embedded steel handle!
end of the wing, and they
should match each other
fairly closely (even though
Steve Wittman was known
to have flown lengthy research and testing hours
with two completely different tips on Buttercup).
Making these wingtips
freehand, or "in space,"
requires some kind of a
rigid mock-up or "buck"
to fit the formed pieces
to. Once the i n d i v i d u a l
halves are made to fit the
mock-up, they must then
be oriented correctly for
welding to avoid twisting
or warping off the plane
of the w i n g . Once the
w e l d i n g jig is done and
that first pair of tips has
been completed, things
settle into a smooth flow
if more are needed. But
getting the first pair done,
that can be frustrating.
Let's bypass the process
of making a pair freehand
Sport Aviation
95
Craft & Technique
and go on to a method of construction that involves making
only one receiver die and a single
welding jig for one set of tips. I'll
be working from a new Sonex
Fiberglas wingtip. No design problems, no development headaches,
and this one tip flips to fit either
side. I just have to build the three
forms from a single existing part. I
begin by making the welding jig.
This part is easy because all I need
is a rigid form with which to assemble and hold the two halves for
welding. I thoroughly wax the inside of the plastic part and then fill
it with concrete or "patch all," a
fast-setting cement-type material.
Adding a welded steel handle while
the mix is wet should enable me to
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hang onto the thing, as 1 expect it to
escape me like a greased pig otherwise. The D-shaped handle is a 3/8inch mild steel round bar welded to
a 3/4-by-3/4-inch angle iron. The
angle is embedded into the concrete, providing a solid anchor for
the exposed handle.
As discussed in the September
"Craft & Technique," I next make
up the receiver into which 1 will
pound the sheet to make one-half of
the wingtip. As before, 1 hot bend
some 1-by-l-inch angle iron to
roughly match the outer edge of the
part, keeping it flat along the upper
flange where the one-half of the tip
will mate to the other. Any warping
will yield an ill-fitting pair of halves,
something even a 1/4-inch "gap"
rod might not fill when welding
them together.
Once the angle is bent and trued
to lie flat, I cobble up a crude box to
retain the mushy mix of plastic necessary for the molding operation.
Don't worry about small leaks or
gaps in the box construction because
the first smear of mud will close
them. I use mild steel anywhere from
18- to 20-gauge in thickness, and I
am not particular in which welding
method I use to join the parts.
Once the box is finished I add a
pair of indispensable horizontal legs,
one at each end of the box and beneath it to enable easy and essential
clamping to the workbench. I finish
by sandblasting the box completely,
and then by making its mate.
By mixing the filler in pint containers I can gradually build up the
obvious voids between the part and
the cavity. I make a first pass with
the plastic, filling the gaps in the
box and making preliminary radius
edges that are more easily filled to a
good density. Checking with the
part installed enables void filling
early on and sets me up for an accurate final pour.
Warm days allow for less added
hardener, if my mixing and pouring
speed is meant to be done at a re-
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Craft & Technique
laxed pace. Additionally, less hardener makes for a stronger mold. I
am using good auto body filler,
which would not last long at all
with hard steel forming tools, but it
holds up well with some specially
molded urethane forming inserts.
When I have determined the cavity is filled reasonably well by check-
ing it with the wingtip, 1 then mix
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The finished flow-formed tip halves are
smooth ready-to-weld mirror images,
and the tooling is ready for more.
up a half-gallon of filler in a gallon
can and dump it into the cavity.
The welding jig is loaded into the
Fiberglas wingtip, the outside of the
wingtip is waxed so it will release
from the plastic filler, and the nowheavy and slippery tip is smushed
down into the bed of wet filler. I
wait until the filler is rubbery before
trimming the excess; wading in too
early just smears it all over the mold,
the bench, and the tools.
The part lifts easily (thank you wax
and handle), and I clean the mold
with a knife and sandpaper and then
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I begin forming with a 2X gun
that is throttled down to a nice sedate pace. The small 1.25-inch diam-
eter urethane head is used for edge
radii, and I can easily shape it with a
sander to meet any contour requirements. I next form the delicate
stepped edge with a steel roundedchisel tool, using very low air pressure, as the mold will erode if I don't.
Saving the lowest and largest area
for last, I switch to a 3X gun and use
the larger forming head with a lowcrown face. Setting the air pressure
low enough to move the metal without leaving marks, I gently flow the
metal into the mold until it resists
me. When this happens 1 re-anneal
the sheet, re-clamp it in the correct
location, and flow it some more.
Some simple planishing may be
needed on the first part to smooth
out some marks, but the second
should be really nice. I do take a little
time to work the metal more in
places where it w i l l not quite lie
down. Sometimes this takes a last an-
neal, and sometimes it takes a little
stretching with a hammer and dolly.
Overall, this is a very satisfying way
to make parts! Next up—wing ribs.
EAA Technical Counselor Kent
White achieved master technician's
status in 1976 at Harrah's Auto Collection, where he restored metal components for aircraft and autos. He
started his own metal restoration company in 1977 and now teaches, writes,
and develops tools for metalworking
while he still pounds out parts. He encourages any welder or metalworker,
man or woman, to contact him in regard to preserving the traditions of aircraft metalworking. To contact him,
call 530/292-3506, e-mail kent@tinmantech.com, or send snail mail to
17167 Salmon Mine Road, Nevada
City, CA 95959.
,
Jd£>
For more information, visit SPORT AVIATION on the Web at www.eaa.org
100
NOVEMBER 2001